The impact of diagenesis on high-precision U [sbnd]Pb dating of ancient carbonates: An example from the Late Permian of New Mexico

Abstract

We have applied U Pb, trace element, and Sr isotopic analyses to subsamples of a single specimen of the Capitan Formation from the Permian Reef Complex of New Mexico with the goal of calibrating the Late Permian time scale and understanding the factors that set and later alter the initial U/Pb ratio and Pb isotopic composition of the limestone. The data indicate a limestone U Pb isochron age of 249.8 ± 4.7 Ma (2σ) and a Pb Pb age of 249 ± 18 Ma. In addition, with μ values (≜ 238U/ 204Pb) ranging up to 4200, among the highest ever recorded for a limestone, we also obtain precise single sample 206Pb */ 238U, 207Pb */ 235U and 207Pb */ 206Pb * ages indicating averages of 250.2 ± 3.2 Ma, 250.0 ± 2.8 Ma, and 256.3 ± 3.5 Ma, respectively. Comparison of the average Pb */U age of 250 ± 3 Ma for the latest Guadalupian with published radiometric constraints for the Late Permian suggests preservation of essentially the depositional age. Trace element and Sr isotopic analyses highlight four samples that preserve a chemical signature expected for primary marine aragonite, including high Sr (> 3000 μg/g), high U (> 2900 ng/g), low initial Pb (< 70 ng/g), low Mg ( < 0.4 mol%), low Fe (< 16 μg/g), low Mn (< 7 μg/g), and 87Sr/ 86Sr identical to contemporaneous seawater (≈ 0.7069). The more altered samples show marked differences in Sr (< 300 μg/g), Mn (> 26 μg/g), and 87Sr/ 86Sr (> 0.7073) as well as the lower U and higher Pb, Mg, and Fe expected for a diagenetically altered carbonate. These data clearly delineate the diagenetic alteration of the limestone and allow us to demonstrate the critical importance of diagenesis in controlling the spread in μ values, primarily through the expulsion of U but with some incorporation of Pb. The high μ are simply a function of preservation of original carbonate chemistry and not of U scavenging by Fe oxyhydroxides or phosphates. Although it has long been suspected that limestone U Pb and Pb Pb isochron ages reflect the timing of early diagenesis, as opposed to a strictly depositional age, this is the first study in which this is conclusively demonstrated to be the case. The U Pb data also suggest a late-stage (Neogene) alteration that disturbed the system mainly via the redistribution of U. Intriguingly, the four samples that best retain their primary marine aragonitic geochemistry also best maintained a closed U Pb system. Most of the subsamples with comparably high μ values but clearly altered trace element compositions showed U Pb systematics indicating late disturbance. Unfortunately it is not yet possible to determine which processes are responsible for this late-stage alteration and it is unclear exactly how to avoid such alteration in future work.